1. Technical Field
The present invention relates to an optical writing display apparatus, an optical writing apparatus, and an optical writing method.
2. Background Art
An optical writing display device displays an image by applying a predetermined voltage to a display device, and controlling a voltage or an electric current applied to the display device by changing the impedance of an optical switching device according to an amount of received light. In particular, optical writing display media, which include a display layer having a memory property and a photoconductive switching layer laminated on the display layer, perform writing upon application of a voltage and an optical image. These media have attracted attention as electronic paper media that can be separated from a writing apparatus and carried.
Examples of a display device having a memory property for the optical writing display medium include polymer-dispersed liquid crystal devices, cholesteric liquid crystal devices, ferroelectric liquid crystal devices, electrophoresis devices, electriefield ball rotation devices, toner display devices, and devices encapsulating these devices.
Examples of an optical switching device capable of controlling a voltage or a current according to an amount of light received include amorphous silicon devices used in electrophotography, photoconductive devices having a function-separated double-layer structure formed from organic photoconductors, and photoconductive devices having a structure in which a charge transporting layer (CTL) is sandwiched between charge generating layers (CGLs) (hereinafter, referred to as a dual-CGL structure). In particular, since photoconductive devices can be produced without a high temperature heating process, they can be formed on a flexible substrate such as a PET film, and since they can be produced without a vacuum process, they can be manufactured at a low cost.
Display devices that use a cholesteric liquid crystal that has a memory property as a material for a displaying layer have also been considered. A cholesteric liquid crystal, which has a helical structure, exhibits a selective reflection phenomenon in which incident light parallel to a helical axis is separated into right-hand circular polarized light and left-hand circular polarized light, and of these, the circular polarization component that coincides with the rotation direction of the helix is Bragg-reflected, while the other is transmitted. The center wavelength λ and reflected wavelength width Δλ of the reflected light are expressed as λ=n×p and Δλ=Δn×p, respectively, where p is the helical pitch, n is the average refractive index in a plane perpendicular to the helical axis, and Δn is the birefringence of the cholesteric liquid crystal. Therefore, light reflected from a cholesteric liquid crystal layer exhibits vivid colors in accordance with the helical pitch.
The cholesteric liquid crystal exhibits the following three states: a planar state in which the helical axis is perpendicular to the surface of a cell and the above-described selective reflection phenomenon of incident light is caused (FIG. 4A); a focal conic state in which the helical axis is substantially parallel to the surface of a cell and incident light is transmitted with a slight forward scattering (FIG. 4B); and a homeotropic state in which the helical structure is unraveled and the liquid crystal director faces in the electric field direction, thereby almost completely transmitting incident light (FIG. 4C).
Among the above three states, the planar state and focal conic state are states of a bistable liquid crystal when no voltage is applied thereto. Consequently, the orientation of cholesteric liquid crystal is not determined only by the level of voltage applied to the liquid crystal layer. When the planar state is the initial state, the liquid crystal changes from a planar state to a focal conic state to a homeotropic state, in that order, as the applied voltage increases, and when a focal conic state is the initial state, the liquid crystal changes from a focal conic state to a homeotropic state as the applied voltage increases.
When the voltage applied to the liquid crystal layer is immediately decreased to zero, the planar state and the focal conic state remain in their respective states, but the homeotropic state changes to the planar state.
Consequently, when voltage is immediately decreased to zero after applying a voltage to the cholesteric liquid crystal layer, the cholesteric liquid crystal layer exhibits a switching behavior as shown in FIG. 5. If voltage is decreased to zero when the partial voltage applied to the liquid crystal layer is Vfh (upper threshold) or more, the liquid crystal is in a selective reflection state in which the homeotropic state has changed to the planar state. If voltage is decreased to zero when the partial voltage is between Vpf (lower threshold) and Vfh, the liquid crystal is in a transmitting state resulting from the focal conic state. If voltage is decreased to zero when the partial voltage is Vpf or less, the liquid crystal maintains its state prior to application of the voltage, namely, the selective reflection state resulting from the planar state or the transmitting state resulting from the focal conic state.
Monochrome optical writing electronic papers, which display a black image on a white background and have been recently developed, include a display medium composed of, between a pair of electrodes, a display layer formed from a liquid crystal layer and a photoconductive layer formed from a photoconductor layer, laminated so as to sandwich a light-shielding layer or the like. In this display medium, a desired image is recorded by exposing the surface of the photoconductive layer side of the display medium to light while applying a predetermined voltage to the pair of electrodes. Specifically, while applying a predetermined voltage to the pair of electrodes, exposure to light is performed, and thereby a photocurrent flows to the photoconductive layer and increases the partial voltage applied to the exposed portion of the cholesteric liquid crystal layer, which changes to a homeotropic state. When the application of the voltage is stopped, the state of the exposed portion changes to a planar state. In this way, an image is written at the display medium.
In the above case, when a voltage is applied between the electrodes, positive holes generated in the charge generating layer of the photoconductive layer move through the charge transporting layer due to the effect of an electric field. Under ideal conditions, positive holes disappear when the voltage application is stopped. However, there are cases in which the positive holes do not disappear because some of these are captured at a trap level, thereby generating a residual potential due to the remaining charges. The effect of the residual potential may prevent the cholesteric liquid crystal layer from changing its state in a desired manner, and satisfactory image writing may not be performed.